Ion implantation is a technique for introducing conductivity-altering impurities into semiconductor and solar cell substrates. During ion implantation, a desired impurity material is ionized in an ion source chamber, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is focused and directed toward the surface of a substrate positioned in a process chamber. The energetic ions in the ion beam penetrate into the bulk of the substrate material and are embedded into the crystalline lattice of the material to form a region of desired conductivity.
Two concerns of the solar cell manufacturing industry are manufacturing throughput and solar cell efficiency. Solar cell efficiency is a measure of the amount of solar energy a solar cell is able to convert into electricity, and is closely tied to manufacturing precision. As technologies advance, higher solar cell efficiencies are required to stay competitive in the solar cell manufacturing industry. Improving precision while maintaining or improving manufacturing throughput is therefore highly desirable.
Ion implantation has been demonstrated as a viable method to dope solar cells in a precise manner. Use of ion implantation obviates processes necessary for existing technologies, such as furnace diffusion. For example, a laser edge isolation process may be removed if ion implantation is used instead of furnace diffusion because ion implantation will not dope areas other than a desired surface. Besides removal of processes, higher efficiencies have been demonstrated using ion implantation. Ion implantation also offers the ability to perform a blanket implant of an entire surface of a solar cell or a selective (or patterned) implant of part of the solar cell. Selective implantation at high throughputs using ion implantation avoids the costly and time-consuming lithography or patterning processes used for furnace diffusion. Selective implantation also enables new solar cell designs.
Micron-level precision may be necessary for the implantation of certain types of solar cells to achieve necessary geometries and tolerances. For example, selective emitter (SE) and interdigitated backside contact (IBC) solar cells have doped regions a few μm apart. If a mask is used to create such doped regions in a cell during ion implantation the locations of the regions are dictated by the placement of the mask relative to the cell. In some cases, the successive introduction of two or more impurity materials into regions of a cell may be desired. This may be achieved by implanting a cell with a first dopant using a first mask at a first ion implantation station, and subsequently implanting the cell with a second dopant using a second mask at a second ion implantation station. In order to implant the first dopant and the second dopant into particular regions of the cell, the first and second masks have to have complementary patterns and have to be aligned with the cell in a nearly identical manner. In other cases, precisely centering a masked dopant pattern on a cell may be desired. As will be appreciated, if precise mask alignment is not achieved in either of the above-described cases (i.e., successive alignment of multiple mask patterns or centering of one mask pattern), cells may not function as desired and/or subsequent processes employed in the manufacture of cells may not be properly aligned.
Any improvements to the precision, reliability, and speed of solar cell manufacturing would be beneficial to solar cell manufacturers worldwide and may accelerate the adoption of solar cells as an alternative energy source.